The present invention relates to a process and apparatus for the separation of foreign matter from a liquid and, in particular, to a process in which a jet of liquid containing induced gas and dissolved gas is used to effect this separation. The process is particularly directed to the separation of oil and water.
Flotation systems which are available at present for the separation of oil and water are of two main types, namely induced gas flotation and dissolved gas flotation. In the induced gas flotation process, gas is introduced into the liquid using a mechanical device such as a rotating impeller which disperses many small gas bubbles into the liquid. Rising gas bubbles then provide a large gas-liquid interface at which oil and oil-wetted particles are concentrated. These particles rise to the surface with the bubbles and remain in the froth above the water from which they can be efficiently and easily skimmed away. The dissolved gas flotation process involves dissolving gas in water at an increased pressure and introducing the water under pressure into a cell maintained at a lower pressure and containing the liquid to be treated. The pressure drop on introduction of the water under pressure into the cell causes small bubbles of the gas to be released. The gas bubbles rise to the surface of the liquid together with the oil to be removed from the liquid. The oil can then be removed from the liquid surface of the cell by skimming.
International Patent Application No. WO85/04432 discloses a means of inducing air into a fluid travelling through a pipe of varying diameter which is divided into inlet, mixing and draught sections.
"Water Research" vol 13, no 4 (1979) pp 385-391 discloses a method of separating oil contaminants from water using dissolved air flotation with a variety of chemical aids.
The difference between the induced gas and dissolved gas flotation processes is, therefore, the method by which the gas is introduced into the liquid. However, both of these flotation processes require considerable amounts of power and generally the apparatus includes several complex and expensive components. For applications in which either space or weight is at a premium, for example on offshore oil field platforms, these flotation systems are very inefficient in terms of floor area, volume and weight.
We have now developed a new process and apparatus for the separation of foreign matter from a liquid by flotation which overcomes the problems and disadvantages of the induced gas and dissolved gas flotation processes, which has a greatly reduced number of components, occupies less space, weighs less and requires less power.